The present invention relates to a plasma X-ray source for producing a high-temperature, high-density plasma by pulse discharge to generate soft X-rays, and more particularly to a plasma X-ray source suitable for use in X-ray lithography for producing submicron integrated circuits or X-ray microscope.
A plasma focus consists of a pair of cylindrical electrodes which are disposed coaxially and insulated from each other by a glass insulator, and is filled with a gas such as deuterium. A pulse voltage from a charged capacitor is applied between the cylindrical electrodes and ionizes the gas, thereby producing a plasma. The plasma thus produced travels between the cylindrical electrodes, and is focused on a position near the open ends of the electrodes. The plasma thus focused is compressed by a pressure of a magnetic field, into a high-temperature, high-density plasma, which generates neutrons. That is, the plasma focus has hitherto been used as a neutron source. However, the high-temperature, high-density plasma formed in the plasma focus also emits soft X-rays. Thus, a plasma focus for a soft X-ray source has been studied, in recent years. A soft X-ray source using a plasma focus is described in, for example, JP-A-No. 60-84749 which is corresponding to U.S. Pat. No. 4,596,030.
An X-ray exposure apparatus and an X-ray microscope require a bright soft X-ray source. In order to use a plasma focus as a soft X-ray source, it is necessary to solve the following problems.
X-rays emitted from a high-temperature plasma which is formed in a plasma focus, are absorbed by a gas contained in the discharge tube and a window thereof, and are then emitted to the outside. In order to emit high-luminance X-rays from the discharge tube, it is required not only to increase the intensity of X-rays generated by the plasma but also to reduce the absorption of X-rays by the gas and window. In order to reduce the absorption of X-rays by the gas in the discharge tube, it is necessary to lower a pressure, at which the discharge tube is filled, and to shorten an X-ray path in the discharge tube. Further, in order to reduce the absorption of X-rays by the window, it is necessary to decrease the thickness of a window material such as beryllium and polymer.